The production of polymer powder by polymerisation reactions of monomers in the presence of catalysts is well-known. For example, processes are known and widely operated commercially using both fluidised bed reactors and slurry phase reactors.
In a slurry polymerisation process, for example, the polymerisation is conducted in a stirred tank or, preferably, a continuous loop reactor in which a slurry of polymer particles in a liquid medium comprising hydrocarbon diluent is circulated. During the course of polymerisation, fresh polymer is generated by the catalytic polymerisation of monomer and polymer product is removed from the reactor by removing a portion of the slurry.
The slurry withdrawn may be treated to separate the polymer particles from the hydrocarbon diluent and other components, such as unreacted monomers, which it is generally desired are recycled to the process.
A slurry polymerisation process generally includes feed systems for fresh monomer and comonomer, as well as for fresh inert liquids. Fresh feeds of monomer and comonomer for example are fed to the polymerisation process to replace monomer and comonomer consumed in the reaction. Although inert liquids don't react they can be lost from the system in process purges or as residual amounts in the polymer passed to downstream storage and processing.
Process purges are required in the system to remove undesired inert components and poisons, which otherwise build up to detrimental levels in the process. Examples include inert hydrocarbons corresponding to the desired monomers and co-monomers. For example, where ethylene is used as a monomer ethane may be present as an impurity in the ethylene feed and can also be produced in the reaction by hydrogenation of ethylene. Other hydrocarbons can be present even where the corresponding monomers are not used. For example methane and propane are often present in low levels in ethylene, and propane can also be present at low levels in isobutane.
It is desired to maintain low levels of such components, although in general the lower the level that is maintained the more other components are lost in the purges. Thus, purging is usually operated to try and maintain a balance in the system between levels of undesired compounds and losses of desired compounds.
The purges are usually applied during the effluent treatment steps in a polymerisation process. In particular, polymer withdrawn from a slurry polymerisation reaction is removed in slurry form in a liquid medium comprising inert diluent, quantities of unreacted monomer and comonomer, and components such as impurities and hydrogen. It is desired to recover the polymer solids essentially free of the other components, and to recycle as much as possible the useful hydrocarbons to the polymerisation reaction.
A common method to achieve this in slurry polymerisation is to heat the withdrawn slurry to vaporise the liquid medium, and to separate the vapour from the polymer solids. This is generally referred to as a “flash”. The vapours can then be condensed and recycled to the reaction, whilst the polymer solids can be recovered for further treatment.
It has become conventional that a first separation step is performed at a relatively high pressure, for example a high pressure flash step. The polymer solids are then usually let down in pressure to a lower pressure second separation step, which may be a lower pressure flash step or a flush step (in which a gas is contacted with the polymer to remove remaining hydrocarbons from the polymer) and remaining hydrocarbons are thereby removed from the polymer.
The pressure and temperature in the high pressure first separation step are generally selected such that the majority of the diluent, monomer and comonomer are recovered in the vapour, and said vapour can be condensed without compression for recycle to the reactor.
The hydrocarbons removed in the lower pressure second step are still present in sufficient quantities that it is economic to recover and recycle them to the process. However, the low pressure second separation step, in contrast to the high pressure recovery system, generally leads to recovered components, such as diluent, monomer and comonomer, which must be compressed (or further cooled) in order to be able to condense them prior to recycle to the reactor.
(“Compression” refers to a process of increasing the pressure (“compressing”) a gas or mixture of gases. This is a relatively energy intensive process. Once in the form of liquids, liquids can be pumped to increased pressure with relatively less difficulty. Avoiding “compression”, for example by condensing without compression, is highly desirable.)
Examples of such systems can be found, for example, in WO 2005/003188 which discloses the use of a higher pressure flash stage followed by a lower pressure flush stage. However, processes are also known where the lower pressure stage is a flash stage rather than a flush stage, or where both flashing and flushing occur in a single stage. (It can be noted that a flush stage can also be referred to as a “purge stage”. The term “flush” is used herein for such steps to avoid any confusion with process purges, which are steps whereby streams are removed from a polymerisation process, for example to flare. The term “purge” as used herein therefore refers to a stream which is removed from the process rather than a flush step.)
Treatments which may be applied to one or both of the separated streams prior to recycle, or at least portions thereof, include treatments to separate components such as “heavy” hydrocarbons and “light” hydrocarbons. The separated heavy and light hydrocarbons are generally purged, usually to flare.
As noted above, the use of a high pressure separation step minimises the compression required to recycle the separated vapours, and it is generally desired that as much of the liquid medium is recovered in this step as possible. Removal of significantly in excess of 90% of the liquid medium is obtainable.